JP7795054B2 - hydrophilic membrane - Google Patents

Description

This application claims the benefit of priority to U.S. Provisional Patent Application No. 62/938,424, filed November 21, 2019, which is incorporated herein by reference in its entirety.

FIELD This application relates to permanently hydrophilic filtration membranes that include a polymeric matrix material and a crosslinked polyoxazoline hydrophilic additive blended throughout the matrix material.

BACKGROUND In many applications of filtration technology, it is desirable to utilize membrane filters that are mechanically strong, thermally stable, relatively chemically inert, and insoluble in most organic solvents. In many cases, membrane surface properties can differ from the bulk properties of the membrane. Desirable surface properties include good wettability, low protein adsorption tendency, controlled ion exchange capacity, and controlled surface chemical reactivity.

One conventional methodology currently used to achieve dual surface property functions is to coat a preformed membrane with an oligomer or polymer that possesses the desired surface properties. Typical coating materials include surfactants and water-soluble polymers such as polyvinylpyrrolidone (PVP). However, this approach has drawbacks. For example, simply impregnating the membrane with a solution of a hydrophilic polymer (to hydrophilize a hydrophobic polymer membrane) is undesirable because the hydrophilic polymer is easily washed away from the membrane.

Over the past few decades, various methods have been developed to permanently modify the membrane surface to improve the surface properties of membrane filters.

U.S. Patent No. 4,698,388 to Ohmura et al. discloses the use of polymeric materials that can be coated with block copolymers. These are synthesized from two types of vinyl monomers in the presence of a polymeric peroxide. One type of polymer can be uniformly dispersed in the polymeric material, while the other type forms a hydrophilic homopolymer. The durability of the hydrophilic modification, characterized by antistatic properties or electrical surface resistivity, was tested only under mild conditions, i.e., by rinsing with tap water at room temperature for two hours. No other mention is made of the permanence of the hydrophilic modification on the polymeric material.

U.S. Patent Application Publication No. 2003/148017 to Tuominen et al. discloses the modification of hydrophobic poly(ether) sulfone-based dialysis membranes by adsorption of a copolymer having hydrophobic polypropylene oxide (PPO) segments and hydrophilic polyethylene oxide (PEO) segments. While washout of the copolymer from the membrane into the dialysate solution is not critical in hemodialysis, it is undesirable in the life sciences industry.

While there are prior patents on polyethersulfone (PES)/poly(2-ethyl-2-oxazoline) (PEOX) membranes (e.g., U.S. Pat. No. 4,900,449 to Kraus et al.) and PES/PVP/PEOX membranes (e.g., U.S. Pat. No. 5,178,765 to Hu et al.), they do not mention further cross-linking of PEOX using an electron beam (e-beam) to create a membrane that is permanently hydrophilic. Furthermore, there is no discussion of methods to reduce extractables/leachables.

U.S. Patent No. 9,045,602 to Thom et al. discloses a method for inducing hydrophilicity on a membrane. This method focuses on modifying the membrane's surface. The process involves impregnating a microporous membrane with a solution containing a solvent and a polymer dissolved or dispersed therein to produce an impregnated membrane. The impregnated membrane is then irradiated with an electron beam, producing a microporous membrane in which the polymer from the impregnation solution is fixed on its surface by crosslinking.

U.S. Patent No. 4,798,847 to Roesink et al. discloses a method for preparing a hydrophilic membrane. The method involves blending a hydrophilic polymer with a hydrophobic polymer and crosslinking the hydrophilic polymer. This method requires that the blended membrane be converted to a non-swollen state prior to the crosslinking reaction.

This application is directed to overcoming these and other deficiencies in the art.

U.S. Patent No. 4,698,388 U.S. Patent Application Publication No. 2003/148017 U.S. Patent No. 4,900,449 U.S. Patent No. 5,178,765 U.S. Patent No. 9,045,602 U.S. Patent No. 4,798,847

SUMMARY This application relates to an unsupported, permanently hydrophilic filtration membrane comprising a polymeric matrix material and a crosslinked polyoxazoline hydrophilic additive blended throughout the matrix material.

A second aspect of the present application relates to a filtration cartridge including a housing and a membrane of the present application disposed within the housing.

A further aspect of the present application relates to a process for forming a permanently hydrophilic filtration membrane. This process involves providing one or more casting solutions of a polymer containing a polyoxazoline and a solvent for the polymer. The casting solution(s) are then applied simultaneously or sequentially onto a support to form a liquid sheet. Phase separation of the casting solution(s) is then allowed to occur in the liquid sheet to form a membrane. The membrane is immersed in water or an aqueous solution. The wet membrane is treated with an electron beam to crosslink the polyoxazoline, and the permanently hydrophilic filtration membrane is separated from the support.

The resulting membranes of the present application, produced by this new process, offer significant improvements in membrane hydrophilicity and stability.

Hydrophilic membranes are made permanently hydrophilic by treatment with e-Beam by crosslinking polyoxazoline throughout the membrane. The e-Beam modification results in permanent hydrophilicity and low extractables/leachables.
[The present invention 1001]
a polymer matrix material;
a crosslinked polyoxazoline hydrophilic additive blended throughout the matrix material;
1. An unsupported permanent hydrophilic filtration membrane comprising:
[The present invention 1002]
The permanently hydrophilic filtration membrane of the present invention 1001, which is a microporous membrane.
[The present invention 1003]
The permanent hydrophilic filtration membrane of the present invention 1001, which is an ultrafiltration membrane.
[The present invention 1004]
1001. A permanently hydrophilic filtration membrane according to claim 10, wherein the extractable total organic carbon of said permanently hydrophilic filtration membrane is less than 20 μg C/cm ² .
[The present invention 1005]
1001. A permanently hydrophilic filtration membrane according to claim 10, wherein the extractable total organic carbon of said permanently hydrophilic filtration membrane is less than 10 μg C/cm ² .
[The present invention 1006]
1001. The permanently hydrophilic filtration membrane of the present invention, wherein the polyoxazoline is selected from the group consisting of poly(2-ethyl-2-oxazoline), poly(2-methyl-2-oxazoline), and poly(2-propyl-2-oxazoline).
[The present invention 1007]
1006. A permanently hydrophilic filtration membrane according to claim 1006, wherein the polyoxazoline is poly(2-ethyl-2-oxazoline).
[The present invention 1008]
1001. A permanently hydrophilic filtration membrane according to the present invention, wherein the polyoxazoline is crosslinked by an electron beam having a dose of about 20-150 kGy.
[The present invention 1009]
The permanently hydrophilic filtration membrane of the present invention 1001, wherein the polymer matrix material is one or more polymers independently selected from the group consisting of PVDF, polyamide, polyimide, polyethersulfone, polysulfone, polyarylsulfone, cellulose ester, polyetherimide, acrylic polymer, methacrylic polymer, and copolymer of acrylic and methacrylic polymer.
[The present invention 1010]
1001. A permanently hydrophilic filtration membrane of the present invention, comprising polyethersulfone and poly(2-ethyl-2-oxazoline).
[The present invention 1011]
The symmetrical permanent hydrophilic filtration membrane of the present invention 1001.
[The present invention 1012]
1001. A permanently hydrophilic filtration membrane of the present invention that is asymmetric.
[The present invention 1013]
1001. A permanently hydrophilic filtration membrane of the present invention having a breaking strain of at least 20%.
[The present invention 1014]
1001. A permanently hydrophilic filtration membrane of the present invention having a breaking strain of at least 25%.
[The present invention 1015]
1001. A permanently hydrophilic filtration membrane of the present invention having a breaking strain of at least 30%.
[The present invention 1016]
A permanently hydrophilic filtration membrane of the present invention 1001 having one or more distinct zones.
[The present invention 1017]
A permanently hydrophilic filtration membrane of the present invention 1016 having three distinct zones.
[The present invention 1018]
1001. A permanently hydrophilic filtration membrane of the present invention having a total thickness of about 65 to 300 μm.
[The present invention 1019]
1001. A permanently hydrophilic filtration membrane of the present invention, which is pleated.
[The present invention 1020]
Housing and
a membrane of the present invention 1001 disposed within the housing;
a filtration cartridge comprising:
[The present invention 1021]
The cartridge of claim 1020, wherein the membrane has an extractable total organic carbon of less than 20 μg C/ ^cm2 .
[The present invention 1022]
1021. The cartridge of claim 1021, wherein the membrane has an extractable total organic carbon of less than 10 μg C/ ^cm2 .
[The present invention 1023]
The cartridge of claim 1020, wherein the polyoxazoline is selected from the group consisting of poly(2-ethyl-2-oxazoline), poly(2-methyl-2-oxazoline), and poly(2-propyl-2-oxazoline).
[The present invention 1024]
1023. The cartridge of claim 1023, wherein the polyoxazoline is poly(2-ethyl-2-oxazoline).
[The present invention 1025]
The cartridge of the present invention 1020, wherein the polyoxazoline is crosslinked by an electron beam having a dose of about 20 to 150 kGy.
[The present invention 1026]
The cartridge of the present invention 1020, wherein the polymer matrix material is one or more polymers independently selected from the group consisting of PVDF, polyamide, polyimide, polyethersulfone, polysulfone, polyarylsulfone, cellulose ester, polyetherimide, acrylic polymer, methacrylic polymer, and copolymer of acrylic and methacrylic polymer.
[The present invention 1027]
The cartridge of claim 1020, wherein the membrane comprises polyethersulfone and poly(2-ethyl-2-oxazoline).
[The present invention 1028]
The cartridge of the present invention 1020, wherein the membrane has a breaking strain of greater than 20%.
[The present invention 1029]
1028. The cartridge of claim 1028, wherein the membrane has a breaking strain of greater than 25%.
[The present invention 1030]
1029. The cartridge of claim 1029, wherein the membrane has a breaking strain of greater than 30%.
[The present invention 1031]
1020. The filtration cartridge of claim 10, wherein the housing comprises an effective membrane area of greater than 0.11 ^m2 per inch of housing height .
[The present invention 1032]
1. A method for forming a permanently hydrophilic filtration membrane, comprising:
providing one or more casting solutions of a polymer comprising a polyoxazoline and a solvent for the polymer;
applying the one or more casting solutions simultaneously or sequentially onto a support to form a liquid sheet;
causing phase separation of the one or more casting solutions in the liquid sheet to form a membrane;
immersing the membrane in water or an aqueous solution;
treating the wet membrane with an electron beam to crosslink the polyoxazoline; and
Separating the permanent hydrophilic filtration membrane from the support
A method comprising:
[The present invention 1033]
1032. The method of claim 1032, wherein the aqueous solution is selected from a mixture of water and one or more multifunctional monomers.
[The present invention 1034]
1033. The method of claim 1033, wherein said one or more monomers are methylenebisacrylamide, Sartomer 9035, tetra(ethylene glycol) diacrylate, and mixtures thereof.
[The present invention 1035]
The method of claim 1033, wherein the membrane has an extractable total organic carbon of less than 20 μg C/cm2 ^.
[The present invention 1036]
1035. The method of claim 1035, wherein the membrane has an extractable total organic carbon of less than 10 μg C/cm2 ^.
[The present invention 1037]
1033. The method of claim 1032, wherein the polyoxazoline is selected from the group consisting of poly(2-ethyl-2-oxazoline), poly(2-methyl-2-oxazoline), and poly(2-propyl-2-oxazoline).
[The present invention 1038]
1037. The method of claim 1037, wherein the polyoxazoline is poly(2-ethyl-2-oxazoline).
[The present invention 1039]
The method of claim 1032, wherein the polyoxazoline is crosslinked by an electron beam having a dose of about 20 to 150 kGy.
[The present invention 1040]
1033. The method of claim 1032, wherein the casting solution comprises polyethersulfone, N-methyl-2-pyrrolidone, triethylene glycol, and poly(2-ethyl-2-oxazoline).
[The present invention 1041]
The method of claim 1032, wherein the membrane has a breaking strain of greater than 20%.
[The present invention 1042]
1041. The method of claim 1041, wherein the membrane has a breaking strain of greater than 25%.
[The present invention 1043]
The method of claim 1042, wherein the membrane has a breaking strain of greater than 30%.
[The present invention 1044]
The method of claim 1032, wherein the polymer is one or more polymers independently selected from the group consisting of PVDF, polyamide, polyimide, polyethersulfone, polysulfone, polyarylsulfone, cellulose ester, polystyrene, polyetherimide, acrylic polymer, methacrylic polymer, and copolymer of acrylic and methacrylic polymer.
[The present invention 1045]
pleating the membrane
The method of the present invention 1032 further comprises:
[The present invention 1046]
1033. The process of claim 1032, wherein said casting solution has an upper critical solution temperature, and said step of inducing phase separation is carried out by cooling said solution below its upper critical solution temperature.
[The present invention 1047]
1032. The process of claim 1032, wherein said casting solution has a lower critical solution temperature, and said step of inducing phase separation is carried out by heating said solution above said lower critical solution temperature.
[The present invention 1048]
1032. The method of claim 1032, wherein the step of inducing phase separation is carried out by vapor-induced phase separation.
[The present invention 1049]
1032. The process of claim 1032, wherein said casting solution further comprises one or more porogens independently selected from the group consisting of formamide, alcohol, polyhydric compounds, water, polyethylene glycol, calcium chloride, and lithium chloride.
[The present invention 1050]
1032. The method of claim 1032, wherein the steps of applying the casting solutions are performed sequentially.
[The present invention 1051]
1032. The method of claim 1032, wherein the steps of applying the casting solutions are performed simultaneously.
[The present invention 1052]
The method of claim 1050, wherein the step of applying said casting solution is performed by co-casting.

FIG. 1 is a perspective view, partially in section, of an exemplary filter cartridge of the present application. FIG. 2 is a cross-sectional perspective view of an exemplary filter cartridge of the present application positioned within a housing in use. FIG. 1 is a side view of an apparatus useful in carrying out the process for producing the permanently hydrophilic membrane of the present application.

DETAILED DESCRIPTION This application relates to an unsupported, permanently hydrophilic filtration membrane comprising a polymeric matrix material and a crosslinked polyoxazoline hydrophilic additive blended throughout the matrix material.

Membranes can be classified as hydrophilic or hydrophobic. When hydrophilic membranes are placed in contact with water, they spontaneously wet, i.e., the water expels air from the pores of the membrane structure without the application of an external force. On the other hand, a positive pressure is required for water to enter the pores of a hydrophobic membrane structure and expel air.

Wetting time is used to characterize membrane wettability in water. It can be determined by placing a drop of 10% NaCl solution on the membrane surface and measuring the time in seconds until the membrane is completely wetted. This can be observed visually as the membrane becomes transparent as it becomes completely wetted.

The permanently hydrophilic filtration membrane of this application may be a microporous membrane or an ultrafiltration membrane.

Ultrafiltration and microporous membranes are used in pressure-driven filtration processes. Experts in the field of membrane-based separation processes readily distinguish between microporous and ultrafiltration membranes and generally distinguish between them based on their applications and aspects of their construction. Microporous and ultrafiltration membranes are manufactured, sold, and used as separate and distinct products. While there is some overlap in nomenclature, they are separate entities and are treated as such in the commercial world.

Ultrafiltration membranes are primarily used to concentrate or diafilter soluble macromolecules such as proteins, DNA, starch, and natural or synthetic polymers. In most applications, ultrafiltration is accomplished by tangential flow filtration (TFF), in which a feed solution is passed across the membrane surface; molecules smaller than the membrane's pore size pass through (the filtrate); the remainder (the retentate) remains on the first side of the membrane. Fluid also passes through and must be recycled or added to the retentate stream to maintain efficient TFF operation. One advantage of using the TFF approach is that the fluid constantly scrubs the membrane's surface, which tends to reduce fouling and polarization of solutes at and near the membrane surface, leading to longer membrane life.

Microporous membranes are primarily used in dead-end filtration to remove particles such as solids, bacteria, and gels from liquid or gas streams. Dead-end filtration refers to filtration in which the entire fluid stream being filtered passes through the filter with no recycle or retentate flow. Any material that does not pass through the filter is left behind on its top surface.

For purposes of this application, ultrafiltration membranes, as compared to microporous membranes, are defined based on the International Union of Pure and Applied Chemistry (IUPAC), "Terminology for Membranes and Membrane Processes," published in Pure Appl. Chem., 68: 1479 (1996), which is incorporated herein by reference in its entirety. Microfiltration is a pressure-driven, membrane-based separation process that rejects particles larger than 0.1 μm and dissolved macromolecules. Ultrafiltration is a pressure-driven, membrane-based separation process that rejects particles smaller than 0.1 μm and larger than about 2 nm and dissolved macromolecules.

The permanent hydrophilic filtration membranes of the present application can be symmetrical or asymmetrical.

As used herein, "symmetric" refers to a membrane or zone having a nearly uniform pore size distribution throughout the membrane or zone.

As used herein, "asymmetric" refers to a membrane or zone in which the pores have diameters that vary continuously or discontinuously through the thickness of the membrane or zone.

For the purposes of this application, a membrane "layer" is defined as an individual membrane sheet. One or more layers of membrane can be used in a filter to remove particles.

In this application, a "zone" is defined as a membrane layer having different physical properties than surrounding or adjacent zones. A membrane layer can contain one, two, three, or even more zones. One or more membrane layers with multiple zones can be used in a filter to remove particles of different sizes.

Similarly, the term "integral" refers to a structure that is made up of multiple zones, and often different polymeric materials, but that is bonded together so as to behave as a single structure and not delaminate or separate under normal use.

The "throughput" of a filter is defined as the amount of fluid that can be processed by the filter until a filtration endpoint is reached. This endpoint can be based on the maximum processing time for filtering a batch or, in the case of constant pressure operation, the minimum filter flux compared to the initial clean water flux. In this application, filter throughput is defined as the maximum volume of fluid that can be filtered by the filter. Because filter throughput depends on the membrane used, membrane throughput is typically measured to predict filter throughput. In the case of constant pressure operation, the volume of fluid filtered per membrane area is the membrane throughput. The maximum volume filtered at 90% of the membrane's initial flux is called V90 and is often expressed in liters per square meter (L/ ^m2 ). Filter or membrane throughput is often interchangeable with filter or membrane capacity.

Historically, asymmetric membranes were developed to achieve high flux rates, which results in improved throughput compared to symmetric membranes. See U.S. Pat. No. 4,261,834 to deWinter, incorporated herein by reference in its entirety. The most common asymmetric membranes have a gradient structure in which the pore size gradually and continuously increases from one side to the other. See U.S. Pat. No. 4,629,563 to Wrasidlo, incorporated herein by reference in its entirety. Another type of asymmetric membrane structure has a retention zone within the asymmetric membrane, where the pore size decreases and then increases again. See U.S. Pat. No. 4,933,081 to Sasaki et al., incorporated herein by reference in its entirety. Advantages of "hourglass"-type asymmetric membranes include high flux rates and reduced risk of loss of retention due to surface scratches.

The permanent hydrophilic filtration membranes of the present application may have one or more distinct zones. For example, the membrane may have three distinct zones.

Generally, membranes with multizone structures are more scientifically attractive because each zone can be fine-tuned to achieve overall improved performance. Multizone microfiltration membranes containing at least one retentive symmetric zone and at least one prefiltration zone were first patented using sequential casting. See U.S. Patent No. 5,620,790 to Holzki et al., incorporated herein by reference in its entirety. As described in U.S. Patent No. 7,208,200 to Kools, incorporated herein by reference in its entirety, sequential casting likely creates a distinct boundary or region with a dense, thin-film-like structure between the retentive symmetric zone and the prefiltration zone. This likely leads to a dramatic reduction in membrane throughput due to particle accumulation at the interface. Furthermore, the symmetric intermediate zone morphology limits its own contribution to the resulting membrane throughput, as described in U.S. Patent No. 5,620,790 to Holzki et al. U.S. Patent No. 7,208,200 to Kools, which is incorporated herein by reference in its entirety, discloses a co-casting process that results in smoother inter-layer transitions and therefore improved performance.

By applying different mix formulations depending on membrane performance requirements, the membrane structure in each distinct zone is precisely tailored. Furthermore, each zone can have its own characteristic morphology; there is no distinct interface zone between any two adjacent casting zones. The membrane morphology can be varied based on different casting formulations and process conditions.

In a further embodiment of the present application, the membrane has a total thickness of 65 to 300 μm.

The membranes of the present application may have a polymer matrix material of one or more polymers independently selected from the group consisting of PVDF, polyamide, polyimide, polyethersulfone, polysulfone, polyarylsulfone, cellulose, regenerated cellulose, cellulose ester, polyetherimide, acrylic polymer, methacrylic polymer, and copolymers of acrylic and methacrylic polymers. Preferably, the membranes of the present application comprise polyethersulfone and poly(2-ethyl-2-oxazoline).

The term "polymer," as used herein, is meant to include polymeric compositions formed from one or more monomers. Representative polymers suitable for forming porous membranes include polyolefins such as polyethylene, polypropylene, polymethylpentene, and the like; polystyrene or substituted polystyrenes; fluorinated polymers including poly(tetrafluoroethylene) and polyvinylidene fluoride; polysulfones such as polysulfone and polyethersulfone; polyesters including polyethylene terephthalate and polybutylene terephthalate; polyamides including poly(hexamethylene adipamide) and poly(phenylene terephthalamide); polyacrylates and polycarbonates; vinyl polymers such as polyvinyl chloride, and polyacrylonitrile. Copolymers such as butadiene and styrene copolymers, fluorinated ethylene-propylene copolymers, and ethylene-chlorotrifluoroethylene copolymers can also be used.

In a further embodiment of the permanently hydrophilic filtration membrane of the present application, the polyoxazoline is selected from the group consisting of poly(2-ethyl-2-oxazoline), poly(2-methyl-2-oxazoline), and poly(2-propyl-2-oxazoline). In a more specific embodiment of the membrane of the present application, the polyoxazoline is poly(2-ethyl-2-oxazoline).

The hydrophilic membranes of the present application can be prepared using an additional hydrophilic polymer(s) in the casting solution. Examples of hydrophilic polymers include polyacrylic acid, polyvinyl alcohol, polyvinyl acetate, polyvinylpyrrolidone, polyethylene glycol, polyvinylpyridine, polyethyleneimine, and polyoxazoline.

In one embodiment, an electron beam (e-beam) is used to crosslink the polymer, for example, by the method described in U.S. Pat. No. 4,944,879, which is incorporated herein by reference in its entirety. Typically, the web or individual samples pass through a curtain of electrons generated by an electron beam processor. The processor delivers the desired dose at an accelerating voltage ranging from 100 kV to 200 kV. Typical dose ranges are from 20 kGy (kilograys) to about 150 kGy.

In a further embodiment of the membrane of the present application, the polyoxazoline can be crosslinked by an electron beam having a dose of about 20 to 150 kGy.

Organic extractables are generally measured as total organic carbon (TOC). In embodiments of all aspects of the present application, the membrane extractable total organic carbon is less than 20 μg C/ ^cm2 , or less than 10 μg C/ ^cm2 .

The polymer casting solutions of the present application typically contain at least one polymer and at least one solvent for the polymer(s). The casting solution may contain one or more components that are poor solvents or non-solvents for the polymer(s). Such components are sometimes referred to in the art as "porogens." The mixes are preferably homogeneous. Optionally, they may contain one or more components that are non-solvents for the polymer. The casting solutions may be stable over time (achieved by good solvent qualities) or metastable over time. The casting solution may also potentially have a lower or upper critical solution temperature. Solvents used include dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, tetramethylurea, acetone, or dimethyl sulfoxide. A myriad of porogens are used in the art, including formamide, various alcohols, and polyhydric compounds such as water, various polyethylene glycols, and various salts such as calcium chloride and lithium chloride.

Other examples of additives include surfactants to further improve wetting, and polymers compatible with the primary membrane polymer used to modify the mechanical properties of the final membrane.

Examples of surfactants used in the preparation of microporous membranes can be found in U.S. Pat. Nos. 4,290,987 to Soehngen et al., 4,298,666 to Taskier, and 4,501,793 to Sarada, which are incorporated by reference in their entireties. Surfactants can be coated onto the membrane as generally known in the art. One such example is disclosed in U.S. Pat. No. 3,853,601 to Taskier, which is incorporated by reference in its entirety. Disclosed polyolefin-based microporous films, such as polypropylene microporous films, are treated with a silicone glycol copolymer surfactant. The microporous film may be impregnated with a combination of a silicone glycol copolymer surfactant and a cationic imidazoline tertiary amine. The surfactant is applied to the polyolefin-based microporous film by contacting the film with a dilute solution of about 1 to 10 weight percent of the surfactant(s) and/or surfactants in an organic solvent, such as acetone, methanol, ethanol, or isopropyl, to provide a surfactant "add-on" to the microporous film of about 2 to about 20 weight percent, based on the weight of the uncoated microporous film.

U.S. Patent No. 4,501,793 to Sarada, incorporated herein by reference in its entirety, discloses the use of alkylphenoxypoly(ethyleneoxy)ethanol surfactants. The surfactants used have an HLB (hydrophilic-lipophilic balance) of approximately 10-15. Such surfactants are well known to those skilled in the art and are readily commercially available. Suitable surfactants include, for example, the 500 and 600 series compounds sold under the Igepal® trademark by GAF Corporation, such as Igepal® RC-520, RC-620, RC-630, CO-520, CO-530, CO-610, CO-630, CO-660, CO-720, CA-520, CA-620, and CA-630. Pluronic surfactants can also be added to increase membrane hydrophilicity.

When comparing the properties of filtration membranes, fluid permeability and bubble point are most commonly used. As used herein, "permeability" is defined as the bulk velocity of a fluid flowing through a membrane at a unit pressure difference across the porous structure, typically measured in liters/( ^m² ·hr·psi). The most common fluids used to measure permeability are air and water.

As used herein, "bubble point" is defined as the pressure of a gas required to displace a liquid from the largest pores of a porous structure. A sample of the material to be tested is immersed in a liquid that naturally fills the pores in the sample. A gas under pressure is then applied to one side of the sample. Initially, the gas does not flow through the sample because the pores within the sample are filled with liquid. However, as the gas pressure increases, the gas will displace the liquid from the largest pores at a certain level of pressure, and the gas will begin to flow through the sample. The pressure at which the gas begins to flow through the sample is known as the bubble point pressure.

The relationship between the size of a fluid-wetted cylindrical pore and the air pressure (P, the bubble pressure in that cylindrical pore) required to empty it is given by
D=4γcosθ/P
where D is the pore diameter, θ is the contact angle, and γ is the surface tension of the wetting liquid. If the measured bubble pressure can be empirically correlated to the size of actual membrane pores, it provides a readily obtainable estimate for the size of actual non-cylindrical pores.

The bubble point of the membrane was measured using a Capillary Flow Porometer (Model No. CFP-1200AEX) manufactured by Porous Materials, Inc. The test liquid used for bubble point measurement was isopropanol.

The breaking strain is defined as the ratio of the membrane's elongation to its original length at the point where it breaks under stress. The breaking strain of membranes was tested using a Zwick/Roell Z2.5 instrument. Test samples measuring 1 inch by 4.5 inches were pre-cut from the left (L), center (C), and right (R) positions of a 10-inch wide membrane roll. The left and right samples were taken from positions 1 inch away from the membrane's edge. The test samples were loaded into the Zwick instrument and stretched under stress. The stress-strain curve was recorded up to the point of break. The reported value is the average of the test results for the L, C, and R samples. In further embodiments of all aspects of the present application, the membrane has a breaking strain of at least 20%, at least 25%, or at least 30%.

In a further embodiment of the present application, the membrane can be pleated without loss of membrane retention. In more specific examples of all aspects of the present application, the membrane is pleated.

As used herein, the terms "pleat" or "pleated" are intended to encompass all such cross-sectional shapes. Relative to the occupied volume, pleated structures present a greater surface area to the incoming fluid process flow than would be presented by the use of a flat sheet. This is particularly advantageous given the desire to maximize device throughput.

The pleats of the membrane can be configured in a corrugated or spirally arranged configuration and can have a looped or folded cross section, such as a W-shaped or M-shaped cross section. Pleated membranes are typically wrapped along their longitudinal axis, with the ends of the pleated membrane sealed together to form a tubular structure or filter tube. In another embodiment, the pleated membrane is sealed within a frame that is sealed to its outer periphery as a flat corrugated filter. In all embodiments, the pleated membrane is designed so that liquid or gas must pass through it to pass downstream.

Membrane retention can be tested using ASTM F838-15 Bacterial Retention Test Method. This test uses worst-case processing conditions to determine the ability of a sterilizing-grade filter to retain a minimum challenge of ¹⁰⁷ cells of Brevundimonas diminuta (B. diminuta) per ^cm2 of filter area. Size controls are run with each test to verify that the test organism is of the appropriate size. For devices made with 0.2 μm pore-size-rated membranes, it is important that the device retains its full capacity when challenged with a B. diminuta solution. If the membrane is too weak, cracks may form during the membrane pleating process. As a result, high diffusion rates are observed within the device, rendering the device less retentive. Crack formation during pleating increases the probability that microorganisms such as B. diminuta will pass through the cracks, thus causing a decrease in retention, or "degraded" membrane retention.

A second aspect of the present application relates to a filtration cartridge comprising a housing and a membrane of the present application disposed within the housing. The filtration cartridge can comprise the membrane of the present application in the form of a filter tube. In one embodiment of the present application, the membrane of the filter cartridge is pleated. In another embodiment, the filter is spirally wound into one or more layers, with or without spacers between them. Furthermore, the cartridge can comprise an effective membrane area of greater than 0.11 ^m2 per inch of housing height.

An example of a filter cartridge of the present application is shown in FIG. 1. The filter cartridge 2 of FIG. 1 includes a pleated high-throughput membrane 4 of the present application, which surrounds a porous hollow core 6 and is provided with a sealing cap 8 and a second cap 10 having an outlet 12. Optionally, an outer porous protective cage is spaced from and surrounds the outer surface of the membrane. Preferably, the cage is sealed to the sealing cap 8 and second cap 10 to form an integrated cartridge filter. The cage, when in use, maintains the membrane in a relatively fixed, tubular conformation. The cage may be made of a rigid material and have uniformly distributed pores, allowing inward flow of fluid from the peripheral region of the pleated filter tube, through the membrane, into the core 6, and then ultimately out the second end cap 10 via the outlet 12.

Further details regarding the structure and function of replaceable filter cartridges are provided by U.S. Patent No. 5,736,044 to Proulx et al. Pleated filter elements can be utilized alone or in conjunction with a prefilter. The prefilter may be positioned within the housing adjacent the fluid inlet or applied to the cartridge adjacent the outer surface of the membrane.

A filter cartridge typically contains a porous filtration element located within a structural housing. In such filters, unfiltered fluid enters the housing through an inlet port and passes through the filtration element, which removes contaminants and other impurities from the fluid. The filtered fluid is discharged through an outlet port. Filter cartridges typically include so-called "quick-change" cartridges, in which the inlet and outlet are combined into a single port at one end of the housing, and in-line cartridges, in which the inlet and outlet ports are located at opposite ends of the housing. Because fluid flow is often pressurized, these ports are typically sealed, for example, with O-rings. Therefore, it is desirable to provide small ports, as larger openings are more difficult to seal (the larger the seal, the greater the force it will experience for a given pressure).

The housing can be constructed as one piece or as two or more components structurally attached together into an assembly. Using a one-piece housing can reduce costs compared to multi-piece assemblies. Polymer one-piece housings can be manufactured by any suitable process, including gas- or water-assisted blow molding or injection molding. Blow molding is generally faster and less expensive than injection molding. Blow molding also stretches and aligns the polymer chains, resulting in stronger and more resilient material properties compared to injection molding. Either process can produce hollow containers, such as plastic bottles, with interiors that have dimensions larger than the desired mouth or port size at the connecting end. Alternatively, the housing can be made of metal and produced by casting or rotational molding processes. While one-piece construction offers cost benefits, the housing can also be manufactured as a two-piece (or more) assembly of injection-molded or cast components.

The filter tube, preferably pleated, is comprised of at least one layer of the high-throughput membrane of the present application. Preferably, the membrane is oriented so that fluid introduced into the housing through the fluid inlet begins its passage through the asymmetric membrane through the open side. Figure 2 illustrates one such design. A cartridge 2 is disposed within a housing 14. The fluid to be filtered, whether liquid or gas, enters the interior of the housing 14 through a first or inlet port 16, as indicated by arrow 26. The fluid passes through the outer surface 18 of the cartridge 2, into the core, as indicated by arrows 20 and 22, and out the second port 12 or outlet of the housing 14, as indicated by arrow 24. If desired, the fluid flow can be reversed, with the second port 12 functioning as the inlet and the first port 16 functioning as the outlet. From the second port 12, the fluid can flow through the core and membrane, past the outer surface 18 of the cartridge 2, into the interior of the housing 14, and out through the first port 16.

A further aspect of the present application relates to a process for forming a permanently hydrophilic filtration membrane. The process involves providing one or more casting solutions of a polymer containing a polyoxazoline and a solvent for the polymer. The casting solution(s) are then applied simultaneously or sequentially onto a support to form a liquid sheet. Phase separation of the casting solution(s) is then allowed to occur in the liquid sheet to form a membrane. The membrane is immersed in water or an aqueous solution. The wet membrane is treated with an electron beam to crosslink the polyoxazoline, and the permanently hydrophilic filtration membrane is separated from the support.

In one embodiment of the process for forming the membrane of the present application, the aqueous solution is selected from a mixture of water and a multifunctional monomer(s), including methylenebisacrylamide, Sartomer 9035, and tetra(ethylene glycol) diacrylate.

In one embodiment of the method for forming a permanently hydrophilic filtration membrane, the polyoxazoline is selected from the group consisting of poly(2-ethyl-2-oxazoline), poly(2-methyl-2-oxazoline), and poly(2-propyl-2-oxazoline). In a more specific example, the polyoxazoline is poly(2-ethyl-2-oxazoline). The ratio of polyoxazoline to PES in the formulation can range from 5 wt% to 50 wt%, and in one example, the ratio of polyoxazoline to PES ranges from 8 wt% to 12 wt%. The polyoxazoline of the membrane can be crosslinked by an electron beam having a dose of approximately 20 to 150 kGy.

The casting solution may further include one or more porogens independently selected from the group consisting of formamide, alcohol, polyhydric compounds, water, polyethylene glycol, calcium chloride, and lithium chloride. In one embodiment of the present application, the casting solution includes polyethersulfone, N-methyl-2-pyrrolidone, triethylene glycol, and poly(2-ethyl-2-oxazoline).

The casting solution may have an upper critical solution temperature, which allows phase separation to occur by cooling the casting solution below its upper critical solution temperature. Alternatively, the casting solution may have a lower critical solution temperature, which allows phase separation to occur by heating the casting solution above its lower critical solution temperature. Additionally, phase separation can also occur by vapor-induced phase separation.

Casting of the permanent hydrophilic membrane can be performed sequentially. Additionally, casting can be performed simultaneously, such as by co-casting.

When forming a multizone membrane, different mixes can be formed for different zones by varying the polymer concentration, solvent or non-solvent, and solution viscosity, additives or treatments, or any combination thereof, to create the desired multizone structure.

The selection of a solvent to provide a stable, homogeneous solution for casting to form a membrane involves the basic principle of polymer solubility. Polymer solvents can be classified as good solvents, non-solvents, and poor solvents. A good solvent is one in which the interaction (force) between the polymer molecules and the solvent molecules is greater than the attractive force between one polymer molecule and another. The opposite is true for non-solvents. A poor solvent is one in which the interaction between the polymer and the solvent is equal to the attractive force between one polymer and another.

In general, zones of a multi-zone structure can be formed from the same polymer and solvent by varying the concentration of components in the mix, viscosity, additives, and processing (before, during, or after formation). Alternatively, different polymers can be used for different zones. If different polymers are used, compatible polymers must be selected. Additionally, the solvent and phase separation material should be the same, if possible, or at least compatible so as not to adversely affect other zones.

There are several processes for preparing porous polymer structures. The most common processes are based on phase separation of polymer solutions. In such processes, the composition or temperature of the polymer solution is changed so that it becomes thermodynamically unstable and separates into two phases. One phase, containing most of the solvent components, is then removed, and the other phase, containing most of the polymer, becomes the porous structure. Phase separation processes typically fall into three categories: 1) vapor-induced phase separation (VIPS), also known as "dry casting" or "air casting," 2) liquid-induced phase separation (LIPS), primarily known as "immersion casting" or "wet casting," and 3) thermally induced phase separation (TIPS), often referred to as "melt casting."

The VIPS and LIPS processes rely on mass transfer between the components of the cast polymer solution, or mix, and the precipitant medium, which exist in the vapor or liquid state, respectively. TIPS is controlled by temperature changes of the polymer solution. The VIPS process has an inherent drawback: low mass transfer rates, which require long residence times in the casting machine, resulting in a trade-off between longer machine lengths and higher capital investments, or slower throughput rates. The LIPS process runs at relatively high speeds due to the higher mass transfer between the polymer solution and the liquid precipitant. This is an advantage in the LIPS process, which is used to manufacture ultrafiltration and reverse osmosis membranes, which require high mass transfer rates to create small pore sizes. However, it presents a complication in processes for creating microporous structures, which typically require moderate mass transfer rates to allow sufficient growth of the dilute phase, resulting in larger pore sizes in the microporosity range of 0.05 to 10 micrometers. Several methods have been devised in the prior art to overcome this complication for creating microporous structures using the LIPS process. One common method is to use a high content of organic solvent, as described in U.S. Pat. Nos. 4,203,847 to Grandine and 4,340,479 to Pall, both of which are incorporated herein by reference in their entireties, or neat organic solvent in an immersion bath. This principle was well analyzed by Wijmans et al., J. Membr. Sci., 14, 263 (1983), which is incorporated herein by reference in its entirety. The drawbacks of this technique are the use of large amounts of flammable organic liquid, which requires explosion-proof manufacturing facilities, and the high cost of solvent disposal.

As taught in U.S. Patent No. 5,444,097 to Tkacik, which is incorporated herein by reference in its entirety, membranes are made from polymer mixes that exhibit a lower critical solution temperature ("LCST") as measured by cloud point. Heating the mix above the LCST causes phase separation. This phase separation step is incorporated into the process of the present application for membrane pore size control. A vapor-induced phase separation step has also been incorporated into the present application. Dew point and vapor temperature affect the membrane pore formation process, and therefore, pore size can be controlled accordingly.

The present process utilizing the LCST process begins with preparing a homogeneous mix of at least one polymer in a solvent system, the solvent system consisting of at least one component that is a solvent for the polymer, where the homogeneous solution exhibits a lower critical solution temperature. The homogeneous mix may optionally contain one or more components that are non-solvents for the polymer. The mixed solution can be prepared by conventional means, mixing the polymer and the components of the solvent system together. The polymer mix is then molded into the desired shape. The molded mix is then heated until phase separation, characterized by cloudiness of the solution, occurs. The components of the solvent system are then removed by methods such as evaporation or extraction. The conditions of the removal process can further affect the later stages of phase separation and the properties of the polymer porous structure. A preferred method for removing the components of the solvent system involves immersing the molded phase-separated polymer mix in one or more liquid baths containing at least one non-solvent for the polymer, where the non-solvent is miscible with at least one component of the solvent system. The porous polymer structure can then optionally be subjected to further extraction or drying.

As taught in U.S. Patent No. 7,842,214 to Romdhane et al., which is incorporated herein by reference in its entirety, vapor-induced phase separation (i.e., air casting) generally involves a coagulating agent (e.g., water vapor) to induce phase inversion. The coagulating agent can be introduced to the polymer material of the membrane as a vapor. High concentrations of vapor can condense and reduce the thermodynamic stability of the polymer material dissolved in the solvent. Similar to liquid-induced phase separation, polymer-rich and polymer-poor regions form from vapor-induced phase inversion, resulting in the formation of a microstructure. Examples of coagulating agents for vapor-induced phase separation include water, alcohols, amides, and combinations thereof.

Contact between the polymer and coagulant vapor at the surface and diffusion of some of the coagulant into the polymer solution can cause the polymer material to become thermodynamically unstable. The polymer material can precipitate from the solution solvent and form microstructures. During phase inversion, regions of the polymer solution layer are rich in polymer material, forming structures, while some regions are poor in polymer material, forming pores. After the microstructures develop, the membrane may be further subjected to solvent removal and subsequent drying.

After the polymer mixes are made, they are applied to a moving carrier. For unsupported membranes, where no web is attached to the final membrane, the carrier is usually a plastic film, such as polyethylene terephthalate, or polyethylene-coated paper, or a similar smooth, continuous web that can be easily removed from the formed membrane.

In one embodiment of the present application, the membrane is fabricated via a slot co-casting process. "Co-casting" means that individual zones are cast onto one another essentially simultaneously, with no substantial time interval between one cast zone and the next. Co-casting is an important aspect of the present application because it allows for the formation of controlled pore size regions at the junctions of the zones. Other casting techniques known in the prior art create clearly defined boundaries between successive cast zones. The dramatic change in pore size, going from a more open to a denser structure, can lead to undesirable rapid particle accumulation at the interface and/or the formation of a thin film layer at the interface, resulting in a dramatic flux reduction. Perhaps due to partial mixing of adjacent co-cast lacquers or high shear forces at the interface between two adjacent co-cast lacquers, the sharp interface can be replaced by a more subtle change in pore size between the two adjacent zones. Such an interfacial zone is beneficial to the membrane's overall structural integrity. At the same time, it allows for the formation of a microporous structure without discernible boundaries within the structure.

Application of the polymer mix can be accomplished by any standard method. The goal is to coat the first mix solution onto the carrier, the second mix solution onto the first mix solution, and the third mix solution onto the second mix solution. A preferred method is co-casting, which is described in detail in U.S. Patent No. 8,123,992 to Kools, which is incorporated herein by reference in its entirety. Co-casting can be accomplished using a triple-knife over-roll apparatus, a pressurized tri-slot coating bead, or any other pre-metered or post-metered coating device known in the industry. Co-casting generally allows for the formation of controlled pore size regions at the junctions of zones; however, even with co-casting techniques, sharp or well-defined boundaries between zones can be formed, if desired, with appropriate selection of materials and application methodology.

Figure 3 illustrates a multiple-zone forming apparatus 28 for casting multi-zone membranes. As shown, the apparatus 28 is designed to produce a three-zone liquid film and has three chambers 30, 32, and 34 containing solutions A, B, and C (one for each zone) to be cast. Additional chambers may be added to form additional co-cast zones as needed. The apparatus includes a front wall 36 and a rear wall 42, with dividing walls 38 and 40 between the front and rear walls. The dividing walls define the volumes of the three chambers. Two side walls complete the apparatus. In operation, the apparatus is fixed on a typical membrane casting machine, and a support web 50 moves or passes under the fixture, dispensing the three solutions through gaps or outlets 44, 46, and 48. The thickness of the zones is controlled by the distance (gap) set between the moving web and the outlet, indicated by gap settings 44, 46, and 48. The final liquid zone thickness is a function of the gap distance, solution viscosity, and web speed. The back wall of the apparatus is typically held a short distance above the support to prevent wrinkling or damage to the support. The back wall gap, support speed, and solution viscosity are adjusted in practice to prevent the solution from leaking through the back wall gap. The apparatus can be fitted with heating or cooling means, either for each chamber separately or for the entire apparatus, as required by solution properties or to further control the final film properties.

A slot die consists of an enclosed reservoir with an exit slot of smaller cross section. An extruder or positive displacement pump, or sometimes a pressurized vessel, delivers the coating at a uniform rate into the reservoir, and all fluid entering the die is forced by pressure out of the reservoir through the slot and onto the moving carrier web. The slot is positioned perpendicular to the moving carrier web. Multiple zone coating requires a die with individual reservoirs, associated feed methods, and an exit slot for each zone.

The membranes of the present application can be manufactured using a pre-metered coating process, in which a precise amount of coating solution to be deposited is introduced into the coating head. The zone height is set by deposition, rather than by some post-application means, such as a doctor blade, which sets the structure thickness after metering the zone (commonly referred to as a "post-metered process"). The term pre-metering applies to die coating, as well as slide and curtain coating, among other methods of forming structures.

After the zones are coated onto the moving carrier, the nascent film is immediately exposed to the environment of a controlled air chamber. Thermally induced phase separation can be initiated by controlled drum temperature, and moisture-induced phase separation can be initiated by moisture absorbed from the air chamber. The nascent film is then immersed in a liquid that is a non-solvent for the polymer and miscible with the solvent and porogen. This causes non-solvent-induced phase separation and ultimately the formation of a porous film. One example of a liquid that can be used as a non-solvent for the nascent film is water.

The formed composite membrane is then typically separated from the carrier and washed to remove residual solvent and other materials. The membrane can then be dried. Water can be used to wash the membrane, and the membrane can be dried in a vacuum drum dryer.

In the coagulation of a multi-zone liquid sheet, coagulation occurs first from the liquid film surface that contacts the coagulation bath, then through subsequent zones of the multi-zone liquid sheet. Each zone dilutes and alters the coagulant as it diffuses through the zone. These changes to the coagulant properties affect the membrane formation in each zone and in the final multi-zone membrane. The thickness, composition, and location of each zone relative to the other zones affect membrane structure and properties. Each zone forms differently than would be formed if made from a single-zone solution or from a stack of single zones.

In another embodiment, the zones are cast sequentially onto a preceding cast. In sequential casting, solutions containing polymers are typically cast, one on top of the other, into thin films, followed by quenching in a non-solvent for the polymers. The first solution is spread in one zone (the lower zone) on a support (such as a non-porous support), the second solution is spread in one zone (the upper zone) on the first solution, and so on. The film can later be separated from the support after quenching; however, the support (porous or non-porous) can be incorporated into the final structure as desired.

The film can be cast manually (e.g., by manually pouring, casting, or spreading the solution onto a casting surface and quenching the liquid applied to the surface) or automatically (e.g., by pouring or otherwise casting onto a moving bed). There should be a time interval between castings of the solution. Preferably, the time interval is about 2 seconds or more. For example, the time interval can be in the range of about 2 seconds to about 35 seconds, or about 2 seconds to about 10 seconds.

Various devices known in the art can be used for casting. Suitable devices include, for example, mechanical spreaders, including application knives, doctor blades, or spray/pressure systems. One example of a spreading device is an extrusion die or slot coater, which includes a casting chamber into which the casting formulation (a solution containing the polymer) can be introduced and extruded under pressure through a narrow slot.

The support with the casting solution thereon is then immersed in a quench bath, resulting in phase separation of the polymer solution. In the quench bath, precipitation or coagulation occurs first from the liquid film surface contacting the bath, then through subsequent zones. After formation, the film is typically washed (e.g., in deionized water) to remove residual solvent and then dried.

The permanently hydrophilic membrane of the present application can be used for dead-end filtration, such as sterile filtration and viral filtration, and tangential flow filtration, such as ultrafiltration, in the life sciences industry. It can also be used in other industrial applications requiring separation media.

If the PEOX in the hydrophilic membranes produced by the process of the present application were not e-Beam crosslinked, the resulting membranes would have a high TOC. Furthermore, the resulting membranes would not have permanent hydrophilicity. The e-Beam crosslinking process makes the hydrophilicity permanent and reduces the TOC without compromising mechanical properties.

Preferences and options for any given aspect, feature, embodiment, or parameter of the technology described herein should be considered to be disclosed in combination with any and all preferences and options for all other aspects, features, embodiments, and parameters of the technology, unless the context dictates otherwise.

The following examples are presented to illustrate various aspects of the present application, but are not intended to limit the scope of the claimed application.

Materials and Methods After the membranes were modified by e-Beam, the membrane performance including TOC, flow time, wetting time, and strain at break were tested.

E-Beam modification
The membranes were modified by exposure to electron beam irradiation (EBLab, Comet Technologies) using an accelerating voltage of 200 kV and a dose ranging from approximately 20 to 150 kGy. The membranes were transported to the e-Beam irradiation chamber at a speed of 3 to 15 m/min. The e-Beam irradiation chamber was inerted with nitrogen. After e-Beam irradiation, the membranes were washed sequentially in methanol and water and dried before further characterization.

Extractables/Leachables Results Extractables/Leachables in water were characterized by the amount of TOC (total organic carbon). One 47 mm disk of each membrane sample was die-cut and autoclaved at 126°C for 1 hour, then soaked in 40 grams of water for 24 hours at ambient temperature. The TOC of the extracted solutions was tested using a Sievers Model 900 TOC Analyzer. TOC standards were prepared using potassium biphthalate, a 1000 ppm carbon stock solution, diluted to 10 ppm carbon with water.

Wetting Time The membranes were baked in air at 135°C for 2 hours and then cooled to ambient temperature before testing the wetting time. The wetting time is determined by wetting the membrane with a 10% NaCl solution according to the following procedure: A drop of 10% NaCl solution (30-60 μL) is placed on the surface of the membrane and the time for wicking of the drop into the membrane is recorded in seconds. The time until the membrane under the drop of NaCl solution is completely wetted is measured.

Flow Time
After e-Beam treatment, the membranes were dried at 70°C for 1 hour and then tested for flow time (the time it takes for 500 ml of water to pass through a 47 mm membrane at -25 in Hg pressure; flow time is inversely proportional to permeability).

Bubble Point Test
25 mm membrane disks were die-cut and used isopropanol (IPA) or Galwick as wetting agents for membrane bubble point testing. The test was performed using a Capillary Flow Porometer (CFP-1200/AEX) manufactured by Porous Materials, Inc. The bubble point is inversely proportional to the pore size and was used to indicate the pore size of the membrane.

Breaking Strain Test
Membrane sample strips measuring 1 inch x 4.5 inches were cut from the left, center, and right sides of the membrane roll, and the breaking strain was measured using a Zwick/Roell Z2.5 instrument. Test samples were pre-cut from the left (L), center (C), and right (R) positions of a 10-inch-wide membrane roll. Left and right samples were taken 1 inch from the end of the membrane roll. The reported values are the average of the test results for the L, C, and R samples. Tests were performed at 23°C and 18-25% relative humidity. 0.98 N was used to check the load cell performance, and the preload was 0.05 N during testing. Flat grips were used with a grip pressure of 30 psi, and the distance between the two grips was 1.5 inches. The test speed was 2 inches per minute. Data were collected as the membrane was stretched to its breaking point.

Example 1 - Permanently hydrophilic membrane using water as soaking agent during e-Beam modification
Permanently hydrophilic membranes with a bubble point of 20 psi (IPA, PMI) were fabricated by triple slot casting using a polyethersulfone and PEOX (Aquazol 500, Polymer Chemistry Innovations, Inc.) blend mix, and then crosslinked by e-Beam irradiation after immersion in water.

Three mixes, including one top, one middle, and one bottom, were prepared according to the formulation in Table 1. The formulation included polyethersulfone (Sumitomo PES 5200P), N-methyl-2-pyrrolidone (NMP), triethylene glycol (TEG), and PEOX. All three mixes were tested. Viscosity and cloud point data are further presented in Table 1. A triple slot die was used to cast a nascent membrane onto a Mylar support above a casting drum. The top mix flow rate was 2.8 L/h, the middle mix flow rate was 33.7 L/h, and the bottom mix flow rate was 5.6 L/h. The nascent membrane was partially formed on the casting drum by controlling the air exposure conditions and drum temperature. It was solidified in a forming bath and then extracted with hot water. The process conditions are shown in Table 2. The membrane was finally dried before performance testing. The cast membrane (RSG2701L) performance, including flow time, bubble point, and breaking strain, was tested. The results are shown in Table 3.

Table 1. Mix formulations for film RSG2701L with cloud point and viscosity data
Note: Viscosity measurements were performed using a Brookfield viscometer (LVDV-II +P) at 35°C with an S62 spindle at 10 rpm.

Table 2: Casting conditions for membrane RSG2701L

The RSG2701L membrane was pre-wetted by immersing it in water and then e-Beam-modified at different doses. The e-Beam-modified membrane was rinsed with methanol, then water, and finally dried before performance testing. The properties of the e-Beam-modified membrane, including TOC, flow time, wetting time, bubble point, and break strain, are listed in Table 3. It is clear that e-Beam modification significantly reduces TOC from approximately 40 to less than 6 μg C/ ^cm² when the e-Beam dose is 50 kGy or higher. Equally importantly, a significant reduction in flow time is also achieved when the membrane is e-Beam-modified. Furthermore, e-Beam modification does not affect the bubble point or break strain within the e-Beam dose range investigated here.

Table 3. Performance of e-Beam modified membrane RSG2701L when using water as the coating solution
- Data not available.

Example 2 - Permanently hydrophilic membrane using N,N'-methylenebisacrylamide solution as soaking agent during e-Beam modification
Permanently hydrophilic membranes with a bubble point of 20 psi (IPA, PMI) were fabricated using a PES and PEOX blend mix by triple slot casting, and then cross-linked by e-Beam irradiation after prewetting with N,N'-methylenebisacrylamide (MBAm, MilliporeSigma catalog no. 146072) solution.

Instead of water, membrane RSG2701L (as prepared in Example 1) was pre-wetted in aqueous MBAm solutions of various concentrations and then modified by exposure to a 50 kGy e-Beam dose. MBAm is a cross-linking agent that can impart hydrophilicity once cross-linked on the membrane's surface. Membrane properties, including TOC, flow time, wetting time, and strain at break, were tested. The results are displayed in Table 4. The incorporation of MBAm can significantly improve wettability but has little effect on TOC and strain. When the MBAm concentration is below 0.4%, the effect on flow time is also small. Permanent hydrophilicity is demonstrated as shown in Table 8 of Example 4.

Table 4. Performance of e-Beam modified membrane RSG2701L when using MBAm aqueous solution as coating solution

Example 3 - Permanently hydrophilic membrane using Sartomer 9035 solution as soaking agent during e-Beam modification
Permanently hydrophilic membranes with a bubble point of 32 psi (IPA, PMI) were fabricated using a PES and PEOX blend mix by triple slot casting, and then cross-linked by e-Beam irradiation after pre-wetting with Sartomer 9035 (SR9035, Arkema Group) solution.

A hydrophilic triple-zone membrane (RSI2318P) with a bubble point of 32 psi (IPA, PMI) was prepared according to the method described in Example 1. Tables 5 and 6 list the mix formulation and casting conditions for this membrane. After casting, the membrane (RSI2318P) was pre-wetted in various concentrations of Sartomer 9035 (SR9035) solution and then modified by exposure to a 50 kGy e-Beam dose. The SR9035 solution improved membrane wettability and had little effect on TOC. Membrane performance data is listed in Table 7.

Table 5. Mix formulations for film RSI2318P with cloud point and viscosity data
Note: Viscosity measurements were performed using a Brookfield viscometer (LVDV-II +P) at 35°C with an S62 spindle at 10 rpm.

Table 6: Casting conditions for membrane RSI2318P

Table 7. Performance of e-Beam modified membrane RSI2318P when using aqueous SR9035 as the coating solution.

Sartomer 9035 concentrations above 0.8 wt% can adversely affect membrane flow. However, the membranes exhibit good hydrophilicity after e-Beam exposure and rinsing with methanol and water. Permanent hydrophilicity is demonstrated as shown in Table 8 of Example 4.

Example 4 - Permanent Hydrophilicity Test Using Methanol Extraction
To confirm whether the e-Beam-modified membranes were permanently hydrophilic, the modified membranes were subjected to Soxhlet extraction with methanol for 48 hours. Table 8 includes the wetting times before and after 48 hours of Soxhlet extraction in methanol. It is clear that the membranes without e-Beam exposure lost their hydrophilicity after methanol extraction, exhibiting a wetting time of more than 30 seconds. This is due to the leaching of PEOX in the membrane during methanol extraction. On the other hand, the e-Beam-modified membranes retained their original hydrophilicity, as indicated by a wetting time of less than 4 seconds after methanol Soxhlet extraction. This indicates that the hydrophilicity of the e-Beam-modified membranes is stable.

Table 8. Membrane wetting times before and after Soxhlet extraction in methanol for 48 hours.

While preferred embodiments have been described and illustrated in detail herein, it will be apparent to those skilled in the art that various modifications, additions, substitutions, etc., may be made without departing from the spirit of the present application and are therefore deemed to be within the scope of the present application as defined in the appended claims.

Claims (7)

1. A method of forming a permanently hydrophilic filtration membrane comprising a polymeric matrix material and a crosslinked polyoxazoline hydrophilic additive blended throughout the matrix material, the method comprising:
providing one or more casting solutions of a polymer comprising a polyoxazoline and a solvent for the polymer;
applying the one or more casting solutions simultaneously or sequentially onto a support to form a liquid sheet;
causing phase separation of the one or more casting solutions in the liquid sheet to form a membrane;
immersing the membrane in water or an aqueous solution;
A method comprising the steps of treating the wet membrane with an electron beam to crosslink the polyoxazoline, and separating the permanently hydrophilic filtration membrane from the support. The method of claim 1 , wherein the aqueous solution is selected from a mixture of water and one or more multifunctional monomers. 2. The method of claim 1 , wherein the polyoxazoline is selected from the group consisting of poly(2-ethyl-2-oxazoline), poly(2-methyl-2-oxazoline), and poly(2-propyl-2-oxazoline). 10. The method of claim 1 , wherein the casting solution has an upper critical solution temperature, and the step of inducing phase separation is carried out by cooling the solution below the upper critical solution temperature of the solution. 2. The method of claim 1 , wherein the casting solution has a lower critical solution temperature, and the step of inducing phase separation is carried out by heating the solution above the lower critical solution temperature of the solution. The method of claim 1 , wherein the step of inducing phase separation is carried out by vapor-induced phase separation. 10. The method of claim 1, wherein the polyoxazoline is crosslinked by electron beam irradiation having a dose of about 20 to 150 kGy.